Many technologies use some type of slip ring/rotary joint when there is a need for continuous contact and/or communication between items that are rotatable with respect to each other. One example is electrical slip rings which have wide spread application. They can be made for high frequency communication or high power applications. Rotary joints are also available for fluids like jet fuel when the hose is unreeled and connected to the wings of a plane.
Fibre optic rotary joints are also available. Commercial products are usually based on cutting of the fibre and inserting lenses and prisms to create an air gap which could allow for rotation of the parts on each side of the air gap. However, the number of fibres in the joints is very limited and the performance is not acceptable for many applications. The loss is fairly high and it is difficult to maintain a stable transmitted signal amplitude during rotation. Back reflection is another limitation of such a concept.
The above state-of-the-art reflects the fact that it is desirable to have a device which could allow for a fairly large number of rotations of one end of a system without causing breakage of cables or undesirable signal disturbances. In applications where only a single or couple of turns is desired, it may be sufficient to allow a cable to allocate such strains, however, if many turns is desirable, it is at present not possible without the sacrifice of signal quality or at the risk of cable damage.
As an example, in the field of offshore seismic surveying activities, a seismic cable is regularly deployed into the sea from a vessel. The seismic cable is typically deployed from a rotatable drum holding the cable. The rotatable drum means that it is difficult to have the cable connected to monitoring equipment during the deployment operation because the rotation inevitably leads to cable torque/twisting which may damage the cable.
In the prior art GB 2424315A describes a solution in which electrical slip rings are provided in a swivel device for allowing electrical signals to be transmitted between a fixed part on a vessel and a cable which may rotate. Further, it is suggested that optical slip rings are known which could achieve a similar function for optical signal transmission members between a fixed part and a rotatable part. Optical slip rings based on the transmission of light over a light transmitting gap, for example in the form of an air gap, between a fixed part and a rotatable part is known. It is a limitation of the device described in GB 2424315A that it is necessary to cut the electrical and optical transmission elements in the cable at the swivel, thus complicating the use of a swivel device with a seismic cable.
The present inventors have not become aware of any previously known rotary joint/swivel device that can achieve this objective for the required number of fibres and thus solve the problem of torque when unloading a continuously connected cable from a rotatable drum without cutting the electrical conductors or optical waveguides to provide for a slip ring solution of the connecting cable, as indicated by the state-of-the-art technology presented in the above mentioned GB-publication.
An alternative technique for avoiding the use of slip rings and rotary couplings in a spooled umbilical system has been disclosed in UK patent no. GB2210355A in which it is described how a cable ribbon is coiled back and forth from a basket to a spool in a spooled umbilical system. The uncontrollable unwinding and folding of the umbilical inside the basket will limit the length of the cable to be handled and represents a risk of a tangle and damage. Although in some applications the umbilical could be shaped in the form of a flat cable to make it easier to handle in a predictable manner, such shaping may in some applications be unpractical or impossible for other reasons.
Hence, a fibre optic swivel that could enable electrical and optical signal transmission through a continuous conductor at a swivel device without cutting the cable and without using slip ring solutions or the like even under many rotations would be advantageous.
In particular, an optical swivel device that do not necessitate the cutting of the cable when handling cables containing optical fibres would simplify the use of such cables when connecting between a fixed structure and a rotatable structure, and improve the operability of such cables.